1. Field of the Invention
This invention relates to a brushless DC motor and more particularly, to a brushless DC motor which does not have a position sensor for detecting a rotational position of a permanent magnet rotor.
2. Description of the Prior Art
Recently, brushless DC motors have been widely used in industrial or audio and video equipment requiring higher reliability for reasons that service life can be improved as well as noise generation can be reduced resulting from such an advantage that they do not need to have a mechanical contact as would be used in conventional DC motors having a brush.
In order to perform the switching operation of a conducting phase the stator windings of a motor, most of conventional brushless DC motors use a rotor position sensor (such as, for example, a Hall effect sensor) instead of using brushes. However the rotor position sensor is not so cheap, and it requires a sophisticated positional adjustment for setting an increased amount of wiring, so that the cost of conventional brushless DC motors is large as compared with DC motors having brushes.
In addition, some structural limitations will be frequently imposed thereupon for the reason that a rotor position sensor has to be set inside the motor itself. A recent trend is that accompanied with the miniaturization of industrial or audio and video equipment, motors to be used are made small in size and thickness, which means that the sectional space where a rotor position sensor such as the Hall effect is to be provided becomes extremely small. Thus, several types of brushless DC motors having no position sensor such as, for example, the Hall effect sensor have been proposed previously.
Out of which, a brushless DC motor disclosed, for example, in Japanese Laid-Open Patent Application No. 55-160980 is based on the so-called half wave driving method in which an electric current is applied unidirectionally to stator windings of the motor. With this method, electromotive forces induced in two stator windings which are stationary out of three-phase stator windings are detected, and the thus detected signals are operationally processed to determine the next conducting phase so as to supply an electric current unidirectionally to the stator windings in a successive manner.
With this method, however, because the rotor is stationary when starting a motor, no counter electromotive force is generated in each of the stator windings. As a result, in such a brushless DC motor as described above according to the prior art, a starting circuit is specially provided for exciting a specific stator winding to thereby determine the initial point of the rotor in advance. In this case, however, even if only one phase of the stator windings is excited in order to determine the initial position of the rotor as shown above, the position of the rotor oscillates and difficult to stabilize, resulting in an increase in starting time.
In addition, the brushless DC motor according to the prior art is based on the half wave driving method in which an electric current is supplied unidirectionally to its stator windings, so that its driving circuit can be made simple in structure on the one hand, but on the other hand, the utility and efficiency of the stator windings are low as compared with a brushless DC motor based on the full wave driving method in which an electric current is supplied bidirectionally to its stator windings, so that a torque to be developed becomes small.
Japanese Laid-Open Patent Application No. 62-260586 discloses a brushless DC motor which is based on the so-called full wave driving method in which an electric current is supplied bidirectionally to its stator windings. An electric current which flows to its stator windings is switched forcibly and successively when starting by a starting pulse signal outputted from a starting pulse generating circuit to thereby drive the motor. When the rotational speed of the rotor is accelerated and counter electromotive forces are induced in the stator windings, the zero-crossing points of the counter electromotive forces are detected thereby to delay its output signal by a constant period of tim a monostable multi-vibrator, thus determining the conduction timing of the electric current.
With the prior art as described above, the stator windings are subjected the forcible and successive switching operation by a pulse signal outputted from a starting circuit. In this case, however, the rotor becomes oscillatory during rotation. As a result, even when the zero-crossing point of each counter electromotive force can be properly detected, it is difficult for the switching to be properly operated from the starting mode to drive the rotor by switching the stator windings forcibly and successively to the normal position detecting mode by detecting the zero-crossing points of counter electromotive forces induced in the stator windings. That is, the timing of switching from the starting mode to the normal position detecting mode of the rotor is difficult technologically, resulting in an increase in starting time of the motor.
In addition, in a brushless DC motor according to the prior art as described above, the conducting phase is determined by delaying a pulse signal generated at the zero-crossing point of a counter electromotive force induced in each stator winding by a constant period of time through a monostable multi-vibrator. In this case, however, the delay time is constant independent of the rotational speed of the motor, which means that it is not suitable for an application in which the rotational speed has to be changed, thus lacking in flexibility of application.
In these brushless DC motors not having a rotor position sensor as described above, the rotor is generally stationary when starting, and no counter electromotive force is generated in each stator winding. As a result, the conducting phase at the initial stage is not allowed to be determined and such a problem has been further pointed out that they are outstandingly inferior in starting characteristics to DC motors having a rotor position sensor.
Also, these brushless DC motors not having a rotor position sensor are considered as a kind of synchronous motor in that the phase switching is forcibly operated when starting, and the frequency for phase-switching suitable for the starting operation is largely varied depending on the magnitude of a load to be applied to the motor or the inertia of the rotor. In some cases, the zero-crossing point of a counter electromotive force induced in each stator winding may not be allowed to be properly detected eternally, so that such a problem has been further pointed out that the switching operation from the starting mode in which the rotor is driven by switching the stator windings forcibly and successively to the normal position detecting mode in which the rotor is driven by detecting the zero-crossing point of each counter electromotive force is difficult to do properly.